For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude. In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes. Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence. Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes. The altitude to which an aircraft may fly is, in many instances, limited to a maximum certified altitude.
As the altitude of an aircraft increases, from its take-off altitude to its “top of climb” or “cruise” altitude, the ambient atmospheric pressure outside of the aircraft decreases. Thus, unless otherwise controlled, air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure at high altitudes. If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity.
Aircraft cabin pressure is often referred to in terms of “cabin pressure altitude,” which refers to the normal atmospheric pressure existing at a certain altitude. Studies have shown that the symptoms of hypoxia may become noticeable when the cabin pressure altitude is above the equivalent of the atmospheric pressure one would experience outside at 8,000 feet. Thus, many aircraft are equipped with a cabin pressure control system functions to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet), allow gradual changes in the cabin pressure altitude to minimize passenger discomfort, and maintain cabin-to-atmosphere differential pressure below nominal and maximum limits. Thus, many cabin pressure control systems control cabin altitude as a function of aircraft altitude, and do so in a manner and rate that will keep the cabin-to-atmosphere different pressure less than the nominal limit.
In addition to an automatic cabin pressure control system, many aircraft additionally include one or more pneumatically-operated positive differential pressure relief valves. These relief valves are provided to limit the cabin-to-atmosphere differential pressure independent of the cabin pressure control system in the unlikely, but postulated, event the cabin pressure control system is inoperable or malfunctions. The pressure relief valves are also useful in the event the aircraft exceeds its maximum certified altitude. This is because present cabin pressure control systems are limited to controlling cabin altitude to, for example, a maximum of 8,000 feet. For example, many cabin pressure control systems implement control logic that controls cabin altitude to a preset maximum altitude (e.g., 8,000 feet) when the aircraft is at its maximum certified altitude. Hence, if the aircraft were to exceed its certified altitude due, for example, to turbulence, the cabin pressure control system would continue controlling, or attempting to control, cabin pressure to the preset maximum altitude, while the positive pressure relief valves prevent the differential pressure limit from being exceeded.
Some aircraft certification authority regulations indicate that certain aircraft should have two independent means to limit cabin-to-atmosphere differential pressure. In some aircraft configurations, this regulation is implemented by providing two independent pneumatic positive pressure relief valves. In other aircraft configuration, this regulation is implemented by including one pneumatic positive pressure relief valve, and relying on the cabin pressure control system as the second means of limiting cabin-to-atmosphere differential pressure. In this latter aircraft configuration, if the aircraft exceeds its maximum certified altitude, then only one independent means of providing positive differential pressure relief is available, since the cabin pressure control system, as was noted above, will be controlling cabin pressure to the preset maximum altitude. Thus, the certification authority regulations are not met.
In addition to short excursions above the maximum certified altitude due to turbulence, there are various other reasons why an aircraft may exceed its initially certified maximum altitude. For example, an updated version of an aircraft that was originally designed as a high frequency regional/commuter type of aircraft may be marketed and sold as an executive aircraft, which is operated at a lower frequency. The high frequency commuter aircraft would likely have a lower maximum certified altitude and lower cabin-to-atmosphere differential pressure limit, than would the low frequency executive aircraft. Other examples in which an aircraft may exceed its original maximum certified altitude include certain military aircraft, aircraft used by NASA, aircraft used for weather services, or aircraft used for other special purpose missions. In each of these cases, once the aircraft is re-certified to the higher maximum altitude, either the cabin pressure control system logic would need to be reconfigured to the new differential pressure limit, or a second positive pressure relief valve would need to be installed to meet the above-mentioned certification authority regulations.
Present cabin pressure control systems that are used in conjunction with a single, independent positive pressure relief valve to limit cabin-to-ambient differential pressure are robustly designed and manufactured, and are operationally safe. Nonetheless, as noted above, these systems suffer certain drawbacks. For example, these systems are not configured to provide an independent means of positive pressure relief if the aircraft exceeds, or is recertified to exceed, its original certified maximum altitude. Thus, the control logic should be updated, which can increase costs, or a second positive pressure relief valve should be added, which can increase costs, weight, and result in another opening in the aircraft fuselage.
Hence, there is a need for a cabin pressure control system and method that extends the cabin pressure control logic beyond the original maximum certified altitude of the aircraft, without having to update the control logic. In doing so, the control system will provides an independent means of limiting cabin-to-atmosphere differential pressure above the original maximum certified altitude, thereby alleviating the need to add an additional positive pressure relief valve, which can reduce aircraft cost, and/or weight, and/or number of fuselage openings. The present invention addresses one or more of these needs.